1. Field of the Invention
The present invention relates to a battery system having a chargeable assembled battery.
2. Description of the Related Art
According to a known system using a battery connected to a commercial electric power system, the battery is charged from the commercial electric power system in a specific time zone in which an electrical utility rate (electric power charge) is low because of a time-of-day electric power charging contract or a midnight electric power contract, and the thus-charged rechargeable battery is concentrically discharged in the daytime, particularly in a time zone in which the amount of used power (hereinafter referred to as “power usage”) is maximum, thereby purveying electric power (for example, see JP-A-2006-149037).
There is the following concern about the power usage. That is, the necessary electric power greatly varies in accordance with ambient temperature, a time zone or the like. When the necessary electric power is larger than electric energy to be discharged, the capacity of the rechargeable battery is short, and when the necessary electric power is smaller than the electric energy to be discharged, electric power discharged from the battery reversely flows to the commercial electric power line. Therefore, a method of efficiently discharging the battery while the electric power charged in the battery is matched with the power usage.